Method and apparatus for modifying the histogram of an image

ABSTRACT

A method and corresponding computing device and computer readable storage media containing instructions for modifying the histogram of a grayscale image to improve contrast by extracting black connected components from the grayscale image that touch at least one of the margins of the grayscale image, computing the histogram of the portion of the grayscale image covered by the extracted black connected components, and updating the histogram of the grayscale image by subtracting the histogram of the portion of the binary image covered by the extracted black connected components from the histogram of the grayscale image or by subtracting a function of number of pixels of the portion of the binary image covered by the extracted black connected components from the histogram of the grayscale image. The function may be a property of a document containing the grayscale image, such as the size of the document. In this case, pixels outside of the document boundaries are removed from the histogram during the updating of the histogram.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 61/025,922 filed Feb. 4, 2008. Thecontents of that provisional patent application are hereby incorporatedby reference.

TECHNICAL FIELD

This invention relates generally to image processing. More specifically,the invention relates to enhancement of image characteristics such ascontrast.

BACKGROUND

Techniques are known for removing noise from digital representations ofdata images obtained by digitally scanning documents and the like. Thescanned documents are processed to identify objects within the scannedimages that are, in turn, used to mask out the noise. For example, U.S.Pat. No. 7,016,536 discloses a method and apparatus for removing noiseby building objects from reduced resolution representations of thescanned image and including the identified objects in a mask that islogically ANDed with the de-skewed representation of the scanneddocument. Objects identified as picture objects are included in a maskand logically ANDed with the de-skewed representation to eliminate allother objects, while objects marked as data objects are added to therepresentation to provide a de-skewed, de-speckled representation of thescanned document.

Digital signal processing techniques are well-known that may be used toprovide contrast enhancement. For example, Pratt discloses in “DigitalImage Processing,” Third Edition, pp. 253-262, a traditional method forcontrast adjustment. Unfortunately, the dark scanning borders that mayoccur around the border area where the scanner sensor was not covered bypaper create spikes in the histogram of the image, which makes suchtraditional contrast adjustment unreliable. A method and digital signalprocessing apparatus are desired that improves the quality of thecontrast adjustment for documents with a dark scanning border. Thepresent invention satisfies such needs in the art.

SUMMARY

The invention satisfies the needs in the art by modifying the histogramto remove the dark scanning border of a scanned image before the imageis processed by a contrast adjustment algorithm. This is accomplished bymodifying the histogram to reduce the number of black pixels caused bythe dark scanning border.

An exemplary embodiment of the invention includes a method of modifyingthe histogram of a grayscale image to improve contrast by extractingblack connected components from the grayscale image that touch at leastone of the margins of the grayscale image, computing the histogram ofthe portion of the grayscale image covered by the extracted blackconnected components, and updating the histogram of the grayscale imageby subtracting the histogram of the portion of the binary image coveredby the extracted black connected components from the histogram of thegrayscale image or by subtracting a function of number of pixels of theportion of the binary image covered by the extracted black connectedcomponents from the histogram of the grayscale image. The exemplaryembodiment also may include binarizing the grayscale image using apredefined threshold to get a binary image and then extracting the blackconnected components from the binary image. The function may be aproperty of a document containing the grayscale image, such as the sizeof the document. In this case, pixels outside of the document boundariesare removed from the histogram during the updating of the histogram.

In any of these embodiments, the contrast of the grayscale image may beadjusted using a contrast adjustment program in which the p_(max) andp_(min) for the pixels of the contrast adjusted image are calculatedfrom the updated histogram h(i) as follows:

$p_{\min} = {{\underset{0 \leq p \leq {R - 1}}{\text{argmax}}p} \in \{ {x❘{{\sum\limits_{i = 0}^{x}{h(i)}} \leq ( {c\mspace{11mu}{\% \cdot N}} )}} \}}$$p_{\max} = {{\underset{0 \leq p \leq {R - 1}}{\text{argmin}}p} \in \{ {x❘{{\sum\limits_{i = x}^{R - 1}{h(i)}} \leq ( {c\mspace{11mu}{\% \cdot N}} )}} \}}$where c is a given percentage value, N is the total image pixel number,and R is the grayscale resolution. The contrast adjustment I_(adj) maybe calculated for each pixel (i,j) as:

${I_{adj}( {i,j} )} = \{ \begin{matrix}0 & {{I( {i,j} )} \leq p_{\min}} \\\frac{( {R - 1} )( {{I( {i,j} )} - p_{\min}} )}{p_{\max} - p_{\min}} & {p_{\min} < {I( {i,j} )} < p_{\max}} \\{R - 1} & {{I( {i,j} )} \geq p_{\max}}\end{matrix} $whereby c % of image pixels are saturated at low and high intensities.Typically, R is 256.

The scope of the invention also includes computer readable storage mediacontaining instructions that when processed by a processor cause theprocessor to implement such methods, as well as a processing deviceprogrammed by such instructions to implement the methods of theinvention. All such features within the scope of the invention willbecome apparent from the following description in connection with theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of a portable scanning systemthat may be adapted to provide contrast enhancement using the techniquesof the invention.

FIG. 2 illustrates a functional block diagram of an exemplary system forcapturing, storing and processing documents for image processing usingthe techniques of the invention.

FIG. 3 illustrates a flow chart of image processing techniques forenhancing and binarizing gray scale images in accordance with theinvention.

FIG. 4 illustrates a scanned document image with a black border to beremoved in accordance with the image processing techniques of theinvention.

FIG. 5 illustrates the histogram of the image of FIG. 4 before it hasbeen modified in accordance with the methods of the invention.

FIG. 6 illustrates the flowchart of the method of modifying thehistogram of an image in accordance with the methods of the invention.

FIG. 7 illustrates the histogram of the image of FIG. 4 after it hasbeen modified in accordance with the methods of the invention.

FIG. 8 illustrates the scanned document image of FIG. 4 after a contrastadjustment in accordance with the invention was performed.

FIG. 9 illustrates a receipt before binarization in accordance with themethods of the invention.

FIG. 10 illustrates the method of calculating the optimum binarizationthreshold in accordance with an embodiment of the invention.

FIG. 11 illustrates the graphical user interface used to determine theoptimal binarization threshold for an individual image.

FIG. 12 illustrates the graphical user interface used to determine whichimages from a collection of images have the optimal binarizationthreshold.

FIG. 13 illustrates a method of binarizing an edge image in accordancewith an embodiment of the invention.

FIG. 14 illustrates a graph of the manually determined thresholds andthe thresholds calculated using the binarization threshold method of theinvention.

FIG. 15 illustrates the scanned image of a receipt for despeckling inaccordance with an embodiment of the invention.

FIG. 16 illustrates the image of the receipt of FIG. 15 after edgedetection and binarization.

FIG. 17 illustrates the image from FIG. 16 after reduction of the numberof speckles in the image using the method of an embodiment of theinvention.

FIG. 18 illustrates a close-up of the receipt image from FIG. 15.

FIG. 19 illustrates a close-up of the edge image from FIG. 16 that hasbeen inverted for illustration purposes.

FIG. 20 illustrates a close-up of the image from FIG. 19 after it hasbeen despeckled by the methods of the invention.

FIG. 21 illustrates the noise candidate speckles superimposed on theclose-up of the original image from FIG. 15.

FIG. 22 illustrates the histogram of the parts of the image of FIG. 15covered by the noise candidates.

FIG. 23 illustrates a flow chart of the method of despeckling the imagein accordance with the invention.

FIG. 24 illustrates the original document image, a receipt, forbackground color calculation in accordance with an embodiment of theinvention.

FIG. 25 illustrates a section of the original document image of FIG. 24.

FIG. 26 illustrates the text mask of the section of the document imageof FIG. 25.

FIG. 27 illustrates the dilated text mask of the section of the documentimage of FIG. 25.

FIG. 28 illustrates the text boundary mask of the section of thedocument image of FIG. 25.

FIG. 29 illustrates the text boundary mask superimposed on the sectionof the document image of FIG. 25.

FIG. 30 illustrates the part of the original image of FIG. 24 that iscovered by the text boundary mask.

FIG. 31 illustrates the histogram of the image from FIG. 29 without theblack colored pixels.

FIG. 32 illustrates the text of FIG. 25 that has gone through contrastadjustment in accordance with the invention.

FIG. 33 illustrates the text of FIG. 25 that has gone through contrastand background color adjustment in accordance with the invention.

FIG. 34 illustrates the image from FIG. 32 that has been binarized.

FIG. 35 illustrates the image from FIG. 33 that has been binarized.

FIG. 36 illustrates the full image of the receipt of FIG. 24 with thebackground color adjusted.

FIG. 37 illustrates the histogram of the original image from FIG. 24.

FIG. 38 illustrates the histogram of the contrast adjusted image asexemplified in FIG. 32.

FIG. 39 illustrates the histogram of the contrast adjusted image with anew background color as shown in FIG. 34.

FIG. 40 illustrates the histogram of the image from FIG. 34 after beingcontrast adjusted once more.

FIG. 41 is a simplified flow chart of an embodiment of the backgroundcolor calculation method of the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Preferred embodiments of the invention will be described in detail belowwith reference to FIGS. 1-41. Those skilled in the art will appreciatethat the description given herein with respect to those figures is forexemplary purposes only and is not intended in any way to limit thescope of the invention. All questions regarding the scope of theinvention may be resolved by referring to the appended claims.

Exemplary System

FIG. 1 illustrates a schematic diagram of a portable scanning systemthat may be adapted to scan documents for classification using thetechniques of the invention. As illustrated, sheet-fed scanner 100 isconnected through communication cable 112 to a computing device 102,which may be a desktop or laptop computer, for example. Scanner 100scans a sheet 110, which may be a sheet of paper, a receipt, a businesscard, or the like, and digitizes the text and graphics thereon in aconventional manner. The scanning result is generally a digital imagethat is transferred to the computing device 102 through communicationscable 112. The digital image may then be manipulated by a computerprogram executed by computing device 102. The computer program asexecuted by computing device 102 may implement various aspects of theclaimed method as explained below.

The computing device 102 includes a display monitor 104 on which thescanned image or manipulated image is displayed to users. Computingdevice 102 may optionally include a memory slot 114, a disk drive 116for storing image files and application program files, and a keyboard106 for providing data input. A mouse 108 is also provided to permitexecution of commands by the computing device 102.

In an exemplary embodiment, the computer program executed by thecomputing device 102 of FIG. 1 analyzes the images received from thescanner 100, converts the images to text, analyzes the text data,extracts expense data, and puts the extracted expense data into anexpense report. In another exemplary embodiment, the computer programexecuted by the computing device 102 of FIG. 1 analyzes business card orother documents images received from the scanner 100, converts theimages to text, analyzes the text data, extracts contacts data, and putsthe contacts data into a contacts database. FIG. 2 illustrates afunctional block diagram of an exemplary system of the type described inU.S. Pat. No. 7,069,240 for capturing, storing and processing documents,such as receipts and business cards, for classification using thetechniques of the invention. The contents of that patent are herebyincorporated by reference in their entirety.

FIG. 2 illustrates an embodiment for processing (and classifying) areceipt using the techniques of the invention. As illustrated in FIG. 2,the process begins with a document such as a paper expense receipt 201of the type typically printed by a cash register or other point of saledevice used by merchants. An input device 202, such as scanner 100 ofFIG. 1, creates a digitized representation of the image of the receiptor other document 203. Preferably, the scanned document is encrypted 204to prevent any tampering with the scanned image. For example, aproprietary encryption methodology may be used that can only besubsequently viewed by a compatible viewer 205. After the digital imageis received by the computing device 102, the image data may be convertedinto alpha-numeric strings 206 using software routines such as opticalcharacter recognition (OCR) and/or image character recognition (ICR) forhandwriting recognition. Such software routines are well-known in theart and will not be described further herein. The process of convertingthe image into alpha-numeric data may also include image enhancementtechniques such as noise filtering, contrast enhancement, edgefiltering, sharpening, and other image enhancement techniques such asthose described below. Preferably, the OCR/ICR process also preservessome of the spatial information with respect to where the text is on thereceipt so as to facilitate parsing of the alpha-numeric text at 207 inorder to find particular information. The parsing may identify certaintext such as the date and amount (where the document is a receipt) andalso look for the vendor name. Also, the document may be searched formeaningful terms and values at 208. For example, if the document is areceipt, the receipt may be searched for common terms at 208. Becausemost receipts possess similar information, the software may search thereceipt text for a list of common terms within the receipt text, suchterms including, for example, “amount,” “total amount,” and/or “amountdue.” The receipt text may also be searched for known date formats. Thesoftware may also utilize a static and non-static database of popularvendors at 209 to facilitate the filing of the expense descriptioncategory in an expense report. For example, if the receipt readsMcDonalds, then the software may recognize that McDonalds is arestaurant and automatically select meals from the expense descriptioncategory in the expense report. Once the software locates the termswithin the receipt text, the corresponding dollar amount or otherrelevant data (date, location, etc.) is extracted at 210 and placed intothe appropriate expense report line item category and correlated to anexpense line item at 211 to build an expense report at 212. Theinformation in the expense report may also be imported into otherpopular expense software such as Quicken and MS Money at 213 and may becustomized to interact with specific corporate systems including storageand filing system 214, processor 215 a, Internet connection 215 b, andbackend system 216.

Image Enhancement and Binarization Techniques

Techniques for enhancing and binarizing gray scale receipt images of thetype captured by the system described above with respect to FIGS. 1 and2 will be described herein in the context of exemplary embodiments.However, those skilled in the art will appreciate that the imageenhancement techniques described herein may be extended to theenhancement and binarization of other types of gray scale documentimages with little or no change.

Given a gray scale image with relatively low image quality, enhancingand binarizing such an image is not a trivial task. Especially when theimage has contrast variances in different areas, a difficult trade-offbetween foreground content and background noise must be made. To enhanceand binarize a category of images (e.g. receipt images) make the taskeven more difficult because the same parameters that work perfectly forone image often will not work for another image, and certainly not forall images. Therefore, parameters used to enhance and binarize imagesmust be decided adaptively based on some criteria, often based on imagequality.

FIG. 3 illustrates a flow chart of image processing techniques forenhancing and binarizing gray scale images in accordance with theinvention. The approach can be divided into the following componentsbased on the major functionality of each component: (1) Preprocessing;(2) Text detection; (3) Object image enhancement; and (4) Object imagebinarization. As illustrated in FIG. 3, the image processing techniqueof the invention processes gray scale image 300 by subjecting it topreprocessing at 302, contrast adjustment at 304, edge detection andnormalization at 306, computation of the edge image binarizationthreshold at 308, despeckling based on the original edge image andbinary edge image at 310, detection of the text from the binary edgeimage 312, computation of the background color and creation of anenhanced image at 314, and binarization of the enhanced image at 316. Inexemplary embodiments, the binarized enhanced image is provided tooptical character recognition (OCR) software for further processing.Each component of this process is described in detail below.

Preprocessing

As illustrated in FIG. 3, an input gray scale image 300 is firstsubjected to preprocessing 302 before enhancing the gray scale image.For example, preprocessing at step 302 may include image smoothing forreducing noise. Besides removing image noise, the smoothing operationmay also cause blurry edges and lost details in smoothed images. Toaddress this, in an exemplary embodiment 3×3 Gaussian low-pass filtermasks may be used to smooth the image as follows:

${I_{s}( {i,j} )} = {{{I( {i,j} )} \otimes {g( {i,j} )}} = {\sum\limits_{m = {- 1}}^{1}{\sum\limits_{n = {- 1}}^{1}{{I( {{i + m},{j + n}} )}{g( {m,n} )}}}}}$where I(i, j) and I_(s)(i, j) represent the pixel at position (i, j) inthe original and smoothed images, respectively. The Gaussian filter maskis defined as:

$g = \begin{bmatrix}0.04 & 0.12 & 0.04 \\0.12 & 0.36 & 0.12 \\0.04 & 0.12 & 0.04\end{bmatrix}$Contrast Adjustment

Contrast adjustment at 304 is very useful for improving the quality ofimages with low contrast. Given an image with minimum and maximum pixelvalues p_(min) and p_(max), the contrast of the image can be adjustedbased on the following formula, where I(i, j) and I_(adj)(i, j) are theoriginal and adjusted pixel values at position (i, j), respectively, andthe pixel may have one of R grayscale values (typically R=256):

${I_{adj}( {i,j} )} = \frac{( {R - 1} )( {{I( {i,j} )} - p_{\min}} )}{p_{\max} - p_{\min}}$

However, images of scanned documents, such as image 400 shown in FIG. 4,often contain large areas of black color 401 around the border areawhere the scanner sensor was not covered by the paper. This abundance ofblack color at 520 creates a spike 501 in the histogram on axes 510, 511of the image (see FIG. 5). This spike 501 makes traditional contrastadjustment methods, such as the one disclosed in William Pratt's book“Digital Image Processing” (third edition, pp. 253-262), unreliable. Inother words, in practical situations, the minimum and maximum values areoften not the real minimum and maximum values but decided from the imagehistogram based on a specified percentage. As shown in FIG. 5, inscanned images, because of the scanning background, the histogram almostalways has a peak value at position 0 which means p_(min) is always 0.The impact is that the contrast can not be really improved for the realobject of interest (i.e. text) in the image. Therefore, a method forminimizing the impact of the scanning background has been developed foruse in an exemplary embodiment of the invention.

A method and apparatus providing contrast adjustment that improves thequality of contrast adjustment for scanned documents with a darkscanning border is obtained in accordance with the invention bymodifying the histogram of the picture before the image is processed bya contrast adjustment algorithm. The method of the present inventionaccomplishes this task by modifying the histogram shown in FIG. 5 byreducing the number of black pixels in spike 501. In FIG. 5, the axis510 represents the brightness, and axis 511 represents the number ofpixels in the image with a given brightness. For each black pixel thatappears in the border 501 of the grayscale image 500, the histogramdensity 501 corresponding to the black color is reduced by one. As usedherein, the term “black pixel” is used to denote any pixel withbrightness below some predetermined threshold; similarly the color“black” denotes all shades that have intensity below a predefinedthreshold. In an exemplary embodiment, any pixel having a brightnesslevel less than 42 is considered to be a black pixel. However, it shouldbe understood that other thresholds may be utilized as known to thoseskilled in the art.

An exemplary embodiment of the invention finds all the black connectedcomponents that touch at least one of the four margins of the image.Algorithms for finding connected components in images are commonknowledge for those having ordinary skill in the art of imageprocessing. The number of pixels in these connected components arecounted and the histogram density at the spike 501 is reduced by thatnumber. In an alternative embodiment, the histogram density at the spike501 is reduced by some function of the number of pixels found in theblack connected components that touch at least one of the margins of theimage. In yet another alternative embodiment, the grey scale image 401of FIG. 4 may be binarized with a threshold of 42. Binarization isprocess of mapping gray scale values of an image to binary values (0 or1). Using a threshold of 42 simply means that all the pixels with valuebelow 42 are mapped to 0, and all the pixels with intensity value over42 are mapped to 1. Of course, other thresholds for the color black maybe used in alternate embodiments. The binarized image is then used tofind the connected components that touch at least one margin of theimage.

FIG. 6 illustrates the flowchart of the method of modifying thehistogram of an image in accordance with the methods of the invention.As illustrated in FIG. 6, the method may be implemented as a computerprogram running on the computing device 102, for example. The methodstarts by reading in the grayscale image S1 at 600 and getting thehistogram of image S1 at 601. The histogram of image S1 is called thehistogram H and, in the case of 256 gray scale values, is a 256-bit long1-dimensional array. Global binarization is applied to the image S1 at602 using a predefined threshold T in order to obtain a binary image S2.At 603, all black connected components CC are extracted from the binaryimage S2 in a conventional manner. Then, at 604, a connected componentCC_(i) is taken from the connected components CC for processing. Inparticular, it is determined at 605 whether the connected componentCC_(i) touches one of the four margins of the image and, if so, theimage S3 covered by CC_(i) is obtained from the original image S1 andthe histogram H_(CC) of image S3 is calculated at step 606. At 607, thehistogram value of histogram H is reduced for the black color by thevalue of the counter i as follows:H[j]=H[j]−H _(CC) [j], (j=0, . . . , T−1)In an alternative embodiment, the value of the histogram H for the colorblack is reduced by some function of counter i. In other words, as thevalue of internal counter, i, is increased by the number of pixels inthe connected component that was found, the histogram H for the colorblack is correspondingly reduced. Similarly, the histogram of the imagemay also be modified by reducing the histogram as a function of the sizeof the document whereby pixels outside of the document boundaries areremoved from the histogram. Finally, at 608, the black connectedcomponents CC are checked to see if there is another black connectedcomponent CCi. If so, the steps 604-607 are repeated; otherwise, theprocess ends.

In an exemplary embodiment, the contrast is adjusted using a contrastadjustment program in which the p_(max) and p_(min) for the pixels arecalculated from the modified image histogram in the following way. Givena percentage value c, the image histogram h, and assuming that the totalimage pixel number is N, then p_(min) and p_(max) for the pixels of theimage are decided as follows:

$p_{\min} = {{\underset{0 \leq p \leq {R - 1}}{\text{argmax}}p} \in \{ {x❘{{\sum\limits_{i = 0}^{x}{h(i)}} \leq ( {c\mspace{11mu}{\% \cdot N}} )}} \}}$$p_{\max} = {{\underset{0 \leq p \leq {R - 1}}{\text{argmin}}p} \in \{ {x❘{{\sum\limits_{i = x}^{R - 1}{h(i)}} \leq ( {c\mspace{11mu}{\% \cdot N}} )}} \}}$And the contrast adjustment is actually:

${I_{adj}( {i,j} )} = \{ \begin{matrix}0 & {{I( {i,j} )} \leq p_{\min}} \\\frac{( {R - 1} )( {{I( {i,j} )} - p_{\min}} )}{p_{\max} - p_{\min}} & {p_{\min} < {I( {i,j} )} < p_{\max}} \\{R - 1} & {{I( {i,j} )} \geq p_{\max}}\end{matrix} $which means c % of image pixels are saturated at low and highintensities, where R typically equals 256. This is a standard contrastadjustment technique.

FIG. 7 illustrates the contrast adjusted image using this modifiedhistogram. In FIG. 7, the axis 710 represents the brightness, and axis711 represents the number of pixels in the image with a givenbrightness. As shown, the peak 701 is significantly smaller than thepeek 501 of FIG. 5. All the other parts of the histogram are the same.The modified histogram can now be used as an input for conventionalcontrast adjustment. In the case of FIG. 7, the contrast adjustment wasdone using a linear transformation of the innermost 98-percentile of themodified histogram. This kind of contrast adjustment is commonly used bythose having ordinary skill in the art of image processing. FIG. 8illustrates the scanned document image of FIG. 4 once such a contrastadjustment has been performed.

Edge Binarization Based on Background Noise Estimation

Binarization of an image involves translating grayscale values,typically 0 to 255, into binary values, 0 or 1. A common way toaccomplish this mapping is to pick a threshold whereby all values underthe threshold are mapped to 0 and all values above the threshold aremapped to a 1. In images with little noise, the quality of thebinarization does not depend on the threshold values in that there is awide range of thresholds that can binarize the images with satisfyingresults. On the other hand, images with lots of noise are very sensitiveto the threshold value. FIG. 9 illustrates an image of a document(receipt) that is degraded and is challenging to binarize. An exemplaryembodiment of a binarization threshold determination technique will bedescribed with respect to FIGS. 10-14.

In an exemplary embodiment, edges are detected and used for detectingtext areas and removing noise. Since edges are normalized to the range[0, R−1] (or [0,255] for an image having R=256 gray scale values),several edge detectors can be chosen to give the same result. In anexemplary embodiment, a Prewitt edge operator may be used to detectedges in 4 different directions. The detected edges are normalized tothe range [0, R−1] using the same approach described above regardingcontrast adjustment. However, in this case, the percentage value c is aconstant value 1. Given the percentage value 1, the minimum and maximumedge pixels are determined from the edge histogram and the edges arethen normalized to the range [0, R−1] based on these two values.Normalized edges are then processed for text detection and also savedfor post-processing binary images.

Given a clean image, only those positions where text interlaces withbackground are there non-zero edges. However, because of the noise on areal scanning image, after normalization, edges of noise areas oftenhave a distribution which can be estimated as a Gaussian distribution.Compared with the distribution of real texts, this distribution oftenhas a smaller mean value. The objective of background noise estimationis to determine a threshold which can binarize edges into boolean valuesas described below. On the other hand, the objective of text detectionis to detect text areas from images and to remove background noisesimultaneously. Once the image noise estimation is estimated, theobtained noise information can be used to remove noise.

In order to remove noise, a threshold T is determined based on theestimated noise distribution to binarize the edge image. In this way,most of the noise with relatively low contrast is removed afterbinarization. The way to decide the binarization threshold is based onthe observation that images with little noise are usually not verydependent on the threshold. In other words, there is a wide range ofthresholds that can binarize edges of good images with satisfyingresults. Images with much noise are usually more sensitive to thethreshold. Therefore, in an exemplary embodiment, a number of noisyimages in an image set C were used to simulate the correlation betweenestimated noise distribution and the threshold. The process to decidethe correlation is explained below in connection with alternateembodiments.

As illustrated in FIG. 10, the process of computing the edgebinarization threshold begins by selecting a set of images, C, at step1000 that will be used to establish a relationship between theproperties of the histogram and the optimal binarization threshold.After the images C to be used in this process have been selected, theuser manually determines the optimal binarization threshold T for eachof the images in the set as explained below.

There are two major ways to find the optimal binarization threshold Tfor a given image in accordance with exemplary embodiments. A first wayis a single image-based approach in which a graphical user interface wasdesigned to import one single edge image every time and to display thebinarization result. As shown in FIG. 11, a graphical user interfaceinput means such as slider 1102 with range [T_(min), T_(max)] was set tochange the threshold T continuously, where T_(min) and T_(max) are theminimum and maximum possible thresholds that can be used to binarize theedge image. Once a satisfying binarization result is achieved (a besttradeoff of noise reduction and keeping the target object), the slidervalue (threshold T) is recorded for that image. As shown in FIG. 10,this process is implemented at 1001 by displaying one image, 1101, fromC at a time and to letting the user adjust the binarization threshold Tusing the slider 1102 illustrated in FIG. 11. The user visually inspectsthe image at various threshold values. When the user finds the optimalbinarization threshold T for the given image, the user presses the“submit” button 1104. The current threshold value is displayed in thetext area 1103.

Once the optimal binarization threshold for image I is submitted, thehistogram H of the image I is computed and H (one-dimensional array withlength 256) is modified at step 1002 to ignore the extreme values ofH[0] and H[255] by updating H as:H[0]:=H[1]; H[255]:=H[254]for an R=256 bit binarization. The gray areas on the image areidentified by trimming the histogram H to length L. Pixels with valuesin the range of [0,L−1] are defined as gray areas, and L is found byfinding the maximum value H_(max) of H at step 1003 and by determiningthe length L of trimmed histogram H as:

$L = {{\underset{0 \leq i \leq {R - 1}}{\text{argmax}}{H\lbrack i\rbrack}} \geq \frac{H_{\max}}{F}}$where F is an empirically determined factor used to reduce the maximumvalue of the histogram. Some statistical information such as the mean m,standard deviation s, and variance v of the histogram H of each image ofthe set H[0], . . . ,H[L−1] are then calculated at step 1004. Thethreshold T, the computed values m, s and v, together with the link tothe image, are then stored in a file or database as data set D at step1005. A determination is then made at step 1006 as to whether there isanother image 1101 in set C. If so, the user is then given the nextimage to examine for threshold coding. Once all of the images in theimage set C are coded, a third degree polynomial curve T=a₀+a₁m+a₂s+a₃vis fit to the data set D at step 1007 and the process ends.

A second way to find the optimal binarization threshold T is themultiple image-based approach illustrated in FIG. 12. In this method,the same threshold range [T_(min), T_(max)] is used and a differentthreshold value is applied to all images at the same time to performbinarization. Images with satisfying binarization results are removedand the threshold value stored for those images. The same operation withchanged threshold is iterated until a best threshold value is found forevery image. This process is implemented as shown in FIG. 12 bypresenting the user on display 1200 all the images, 1201, from the imageset C. The threshold value is displayed in the text area 1205. For anR=256 bit binarization, the binarization threshold starts at 0 andcycles to 255. At each threshold value, the user is asked to pick thoseimages that have an acceptable binarization at the given thresholdvalue. The user chooses the images by clicking on the checkbox, 1202,associated with the image. When the user checks a given image, acheckmark appears in the checkbox, 1202 a. This causes the thresholdvalues, together with the link to the image, to be stored in a file ordatabase. When the developer has finished choosing the images that haveacceptable binarizations at the given threshold value, the user canclick on the Next button, 1203, to move on to the next binarizationvalue. The user can also choose to go back to previous threshold valuesby clicking the Previous button, 1204. Once the optimal binarizationthreshold for each image 1201 is submitted, processing may continue withsteps 1002-1007 as described above with respect to the embodiment ofFIG. 11.

FIG. 13 illustrates a method of binarizing an edge image in accordancewith an embodiment of the invention. This method is essentially the sameas the method of FIG. 10 except that, as noted above, the detected edgesare normalized at step 1300 to the range [0, R−1] where the percentagevalue c is a constant value 1. Given the percentage value 1, the minimumand maximum edge pixels are determined from the edge histogram and theedges are then normalized to the range [0, R−1] based on these twovalues. Steps 1301-1303 are the same as steps 1002-1004, respectively.However, in the edge binarization method of FIG. 13, the binarizationthreshold T is calculated at 1304 as T=a₀+a₁m+a₂s+a₃v and each pixel inimage I with a pixel value smaller than T is set to 0 and each pixelwith a pixel value larger than T is set to R−1 at step 1305 to producethe binary edge image at 1306.

The data threshold and histogram data stored in the file or database maybe sorted by the value of the threshold. In such case, the sorted matrixhas the following format:

Standard Threshold Mean Deviation Variance 19 11.2 7.36 54.1696 21 12.157.58 57.4564 32 16.21 11 121 33 18.17 9.62 92.5444 49 23.31 13.5 182.2555 18.99 11.88 141.1344 56 15.52 10.02 100.4004 60 14.85 9 81 65 30.0314.53 211.1209 66 30.1 15.19 230.7361 66 31.97 17.02 289.6804 67 21.4815.24 232.2576 70 23.03 15.04 226.2016 72 24.13 13.58 184.4164 77 21.411.22 125.8884 85 35.59 17.99 323.6401 89 35.08 19.71 388.4841 96 25.0114.23 202.4929 100 31.81 17.03 290.0209 104 43.77 30.11 906.6121 10939.03 23.27 541.4929 115 32.29 16.47 271.2609 140 43.05 23.26 541.0276182 46.28 27.07 732.7849 201 52.93 29.55 873.2025 215 67.37 29.77886.2529 243 69.5 33.96 1153.2816 255 55.25 27.93 780.0849 255 53.9926.35 694.3225 255 86.67 44.69 1997.1961 255 111.64 53.79 2893.3641

As noted above with respect to FIGS. 10 and 13, once the data preparedin this way, the mathematical relationship between the threshold andmean, standard deviation, and variance is constructed by fitting a thirddegree polynomial curve T=a₀+a₁m+a₂s+a₃v to this data set. Curve fittingis common knowledge with those having ordinary skill in mathematicalmodeling. In an exemplary implementation, the vector of coefficientsA=[a₀ a₁ a₂ a₃]^(T) is found using a minimum mean square error algorithm(MMSE). Using the above 31 images as an example, the procedure to fitthe curve is as follows:

-   -   (1) Construct a 31×4 matrix X=[I M S V], where I is an identity        column vector, M is a column vector containing the mean values        of the images, S is the column vector containing the standard        deviations of the images, and V is the column vector containing        the variances of the images.    -   (2) Construct a 31×1 column vector Y containing 31 binarization        threshold values. Find A that minimizes the mean square error        e=(Y−X·A)^(T)·(Y−X·A). For example, in an exemplary embodiment        the MATLAB program was used to implement the MMSE algorithm and        A was computed using MATLAB code:

X = [ones(size(M)) M S V]; A = X\T.

Once the value of A is calculated, the threshold value of any imagebased on the mean (m), standard deviation (s), and variance (v) of theimage histogram may be calculated using the following formula:T=[1 m s v]·AWhen the above algorithm is applied to the data displayed in the tableabove, the resulting values for the A vector are: [−67.306 4.4275 4.2539−0.132]^(T). In other words, for a given image, once statisticalinformation such as the mean (m), standard deviation (s), and variance(v) of the histogram of a given image are calculated, the optimalbinarization threshold can be calculated using the following formula:T=−67.306+4.4275m+4.2539s−0.132vFIG. 14, shows the graph of the threshold values as they are codedmanually and as they are calculated using the above equation.

In the above examples, only 31 noisy images were used to decide thecorrelation between noise edge distribution and edge binarizationthreshold. The decided curve might not be close enough to the realsituation. There is no reason that the decided formula can not bechanged if there are more images available.

The technique described with respect to FIGS. 10-14 reduces the noisesignificantly; however, if the original image contains relatively strongnoise, there will still be a noise remainder. Therefore, furtheroperations are desirable to remove the remaining noise. Such furtheroperations are described below.

Despeckling to Further Reduce Noise

When preparing scanned documents for optical character recognition(OCR), it is important to reduce the noise in the image. Thispreprocessing of the image improves the accuracy of the opticalcharacter recognition. Documents, such as receipts, that are of poorquality pose a special challenge regarding image processing. Moreover,if there exists relatively strong noise on the original image,de-noising relying only on edge binarization is often not enough. In anexemplary embodiment, it is further desirable to evaluate the noiselevel of the image first and perform despeckling to reduce more noise ifnecessary. Generally speaking, this process proceeds as follows.

-   -   1. Extract all edge connected components (white components) from        the binary edge image.    -   2. All small components are potentially noise components and        thus should be recorded for evaluation. In an exemplary        embodiment, for an image scanned using 300DPI, components with        dimension smaller than 5×5 or with pixel number less than 12        were considered noise candidates. Of course, other sizes of        components could be considered noise candidates as desired. For        another scanning resolution, R_(DPI), the dimension threshold        should be changed to

$\frac{R_{DPI}}{60} \times \frac{R_{DPI}}{60}$and the pixel number threshold should be changed to

$\frac{R_{DPI}}{25}$to make them adapt to any scanning resolution.

-   -   3. If the number of stored noise candidate components for the        whole image is large enough (larger than 1% of the total number        of components), then continue the processing to reduce more        noise and refine text areas. Otherwise, stop the processing        here.    -   4. Compute the edge histogram h_(e) of all those noise        candidates using the non-binary but normalized edge image.    -   5. Compute an edge threshold E_(th) from the total number of        noise candidate pixels.    -   6. Go over all pixels covered by noise candidate components and        set pixels with edge smaller than E_(th) to background.

FIG. 15 illustrates an image of a scanned receipt that needs to beprepared for OCR using the despeckling technique of the exemplaryembodiment. FIG. 16 illustrates the image of the receipt of FIG. 15after edge detection and binarization as described above, while FIG. 17illustrates the image from FIG. 16 after reduction of the number ofspeckles in the image using the despeckling method described herein. Itis clear that a significant amount of small noise is present in theimage of FIG. 16 that is not present in FIG. 17. FIG. 18 illustrates aclose-up of the receipt image from FIG. 15. FIG. 19 illustrates aclose-up of the edge image from FIG. 16 that has been inverted forillustration purposes, while FIG. 20 illustrates a close-up of the imagefrom FIG. 19 after it has been despeckled by the methods of theinvention. Clearly, fewer noise elements 2010 are present in FIG. 20than noise elements 1910 present in FIG. 19. FIG. 21 illustrates thenoise candidate speckles 2110 superimposed on the close-up of theoriginal image from FIG. 15, while FIG. 22 illustrates the histogram ofthe parts of the image of FIG. 15 covered by the noise candidates.

FIG. 23 illustrates a flow chart of the method of despeckling the imagein accordance with the invention. Starting at 2301, the edges of theinput image are detected and the edge-detected image is normalized tovalues of 0-255 (for R=256). The result is stored in I. The resultingimage I is then binarized and the results are stored in I_(b). Both ofthese procedures are known in the art of image processing. The procedurefor eliminating the specular noise then proceeds as follows.

First, at step 2302 all the white connected components in the binaryedge image I_(b) displayed in FIG. 16 are found and a list, L, of all ofthe connected components is created. Finding connected components inimages is known in the art of image processing.

Second, all the connected components that are smaller than 5×5 pixels orwith less than 12 pixels are considered as noise candidates at 2303.These are the thresholds used in an exemplary embodiment; otherthresholds for noise candidates can be used when appropriate. The resultis a list, C, of connected components that are considered noisecandidates. So, in the image from FIG. 19, all the speckles in the 1910area are considered noise candidates. Clearly, there are other noisecandidates in this image.

Third, the number of components M in list C is counted and the number ofcomponents N in list L is counted. The percentage of noise candidates Mto the number of all connected components N is computed as thepercentage p=M/N at 2304. If the number of noise candidates M in thelist is greater that 1% of the total number of connected components N at2305, then processing of the noise candidates proceeds. Otherwise, nofurther processing is necessary and processing exits. This step may beeliminated in alternative embodiments of the invention. Also, the 1%threshold can also be modified in alternative embodiments of theinvention.

Fourth, for each noise candidate, the corresponding area of the originalimage that is covered by the noise candidate is found at 2306. Thiscollection of pixels D represents those pixels from the edge image Ithat are covered by the connected components in list C. The number ofpixels in D are denoted by N_(noise). FIG. 21 shows the noise candidatesoverlayed onto the original receipt image. The dots, 2110, are masksthat are used to pick out the relevant pixels from the original image.This step creates a collection of pixels from the original image thatare covered by the noise candidates.

Fifth, at 2307, a histogram h_(e) of the collection of pixels assembledat 2306 is computed. FIG. 22 shows the histogram of the collection ofpixels from 2306. In this histogram, the original gray scale image underthe speckle candidates is pulled out in a mask and the original image ispurged of noise candidates that exceed threshold 2230. In this case, t %of the noise candidates are to the right of the threshold 138.

Sixth, at 2308, the threshold, E_(th), is calculated from the histogramh_(e) using the following formula:

$E_{th} = {{\underset{o \leq E \leq {R - 1}}{\text{argmin}}E} \in \{ {x❘{{\sum\limits_{i = x}^{R - 1}{h_{e}(i)}} \leq ( {t \cdot N_{noise}} )}} \}}$where N_(noise) is the total number of noise candidate pixels, and t isthe threshold that is empirically determined. R is typically 256 but mayvary depending upon the grayscale resolution.

Finally, at 2309, the threshold calculated at 2308 is used to eliminatespecular noise. This is accomplished by examining each of the pixels inthe noise candidates. If corresponding pixels from the original imageare smaller than E_(th) in value, then the noise candidate pixel is setto the background color. The result of the filtering is displayed inFIG. 20. The area 2010 shows that there are no speckles in the areawhere they were present in area 1910 of FIG. 19.

The binary edge image with noise correction as described above is nowready to process to detect the text. The object text areas areidentified and enhanced. Since the detected text areas are well defined,the enhancement of original images can be easily transformed to objectimage enhancement. The approach described below selectively enhances thedetected text area and suppresses the background noise impactsimultaneously.

Create Text and Background Picking Mask

Given a binary edge image as processed above, the text picking mask canbe easily defined using connected components on the binary image.Assuming the given image is the result image right after the textdetection, which is a negative binary image, then the text picking maskconsists of positions of all white pixels on the input image. Thepurpose of the background picking mask described herein is to enablecalculation of a background color that would make textual documents easyto binarize. Not as simple as the creating of text picking mask, the wayto create the background picking mask is a little bit tricky. In orderto suppress the impact of background noise maximally, instead of settingthe background pixel to a white value, the background pixel value iscomputed as follows:

-   -   1. Dilate the input binary edge image S₁ to generate a new        binary edge image S′₁.    -   2. Optionally perform hole filling to all white connected        components on image S′₁ to generate another new image S₂.    -   3. Compute the difference image S_(dif) of S₁ and S₂, where        S_(dif)=|S₁−S₂|.    -   4. All pixels on image S_(dif) with non-zero values will be        considered background pixels.    -   5. Compute the average of all decided background pixel values        from the original input image. This value is the background        value that can be used to generate the object image.

FIGS. 24-41 illustrate an exemplary embodiment of the invention wherebysuch a background picking mask creating procedure is used to select adetected text component “AUTH” by way of example. The same procedurewill happen to each text component on the given image, and the averagebackground pixel values will be used as the final background pixelvalue.

FIG. 24 illustrates the original document image, a receipt, forbackground color calculation in accordance with an embodiment of theinvention. The credit card number and other private information has beenblanked out in this image. The “AUTH” word, 2401, is the word that willbe used to illustrate the implementation of the background picking maskin accordance with the method illustrated in FIG. 41.

FIG. 25 illustrates a close-up of the “AUTH” word as it appears in theoriginal document illustrated in FIG. 24. FIG. 26 shows the text mask ofthe “AUTH” word. This text mask is constructed by detecting the edges ofthe original image and binarizing the detected edge image at 4101. Edgedetection is a commonly known image processing technique known in theart. For example, the open-source image processing software “GIMP” hasmultiple procedures available for edge detection. In an exemplaryembodiment, an “edge filling” and “hole filling” procedure is used at4102 to fill in the gaps that are present in letters such as “A” to getan updated binary edge image S1.

FIG. 27 shows the text mask of the “AUTH” word after it has been dilatedat 4103 to create image S2. Dilatation increases the thickness of thetext mask by a fixed number of pixels. In an exemplary embodiment, thethickness is increased by one pixel. Dilatation is also a commonly usedimage processing technique known in the art. For example, theopen-source image processing software “GIMP” has procedures fordilation.

FIG. 28 shows the text boundary mask. This mask is derived by doing anexclusive or (XOR) operation on the image S1 in FIG. 26 and the image S2in FIG. 27 at 4104. Specifically, each pixel in FIG. 28 is calculated asa XOR of the corresponding pixels in FIG. 26 and FIG. 27. In otherwords, the pixel in FIG. 28 is white if only one of the correspondingpixels in FIGS. 26 and 27 are white. If both pixels are white or bothpixels are black, then the corresponding pixel in FIG. 28 is black. Thismask selects the pixels forming text boundary S3 that surround the“AUTH” text.

FIG. 29 illustrates the text boundary mask S3 superimposed on thesection of the document image of FIG. 25 at 4105. Careful comparison ofFIGS. 28 and 29 reveals that the text boundary mask of FIG. 28 is white,while the text boundary shown in FIG. 29 assumes the color of thebackground of the original image from FIG. 25. Specifically, each pixelfrom FIG. 29 is composed using the following pseudocode:

if p28(x,y) is white then set p29(x,y) to p25(x,y),   otherwise setp29(x,y) to black,where p25(x,y), p28(x,y), and p29(x,y) are the value of thecorresponding pixel at coordinates (x,y) of the image from FIGS. 25, 28,and 29, respectively. FIG. 30 illustrates the part of the original imageof FIG. 24 that is covered by the text boundary mask S3.

FIG. 31 illustrates the histogram of the non-black pixels of the imagein FIG. 29. In other words, this histogram is calculated at 4106 usingonly those pixels covered by the text boundary mask S3. The histogram ofFIG. 31 is used to calculate at 4107 the new background color, a, forthe images by finding the average value of the non-black pixels.Specifically, in the image from FIG. 29, there are 527 non-black pixelsthat represent the background surrounding the text from the image fromFIG. 25. The average value, a, of these pixels is 220. In the exemplaryembodiment, the average value of the non-black pixels of FIG. 29 is usedas the new background color for the image.

FIGS. 32 and 33 illustrate the application of the newly calculatedbackground color a to the image at 4108. FIG. 32 shows a contrastadjusted image from FIG. 25. For example, the contrast adjustment couldbe a conventional contrast adjustment or the technique described indetail above. In FIG. 33, all the background pixels are turned into thecalculated background color, 220 in the current example. Backgroundpixels are all those pixels not covered by the text mask of image fromFIG. 26.

FIGS. 34 and 35 show binarized versions of FIGS. 32 and 33,respectively. It is clear that the amount of noise in FIG. 35 is lessthan the amount of noise in FIG. 34. This difference in noise can leadto significantly more accurate OCR results for FIG. 35 than for FIG. 34.FIG. 36 illustrates the image from FIG. 24 after the background color isadjusted using the technique just described.

Numerous alternative embodiments are also possible within the scope ofthe invention. For example, the methods described herein may be used toenhance images to make it easier for optical character recognition (OCR)software to be more accurate. In order to fully appreciate theseadvantages of the invention, it is useful to examine the histograms ofthe images in various stages of the process described above. Inparticular, FIG. 37 illustrates the histogram of the original image ofFIG. 24. FIG. 38 illustrates the histogram of the contrast adjustedimage as exemplified by FIG. 32. FIG. 39 illustrates the histogram ofthe image exemplified in FIG. 33, which is the contrast adjusted imagewhen the background was modified to the new color calculated in theprocess described above. FIG. 40 illustrates the histogram of the imagefrom FIG. 33 further contrast adjusted. It should be noted that thehistogram profile showed in FIG. 40 is more suitable for binarizationand preparation for OCR processing.

By combining the techniques described above, it becomes apparent thatthe way to generate the best object image for OCR is to combine theenhancement of object texts and to suppress the background. Inparticular, after the selective enhancement of the object image (e.g.“AUTH”), the obtained image is a gray scale image with the followingfeatures:

-   -   Noise is reduced.    -   Contrast is adjusted.    -   Object texts stand out.        To get the best OCR result, before feeding this image to the OCR        engine, a locally adaptive binarization approach may be applied        to create a good binary image.        Various Embodiments

As is apparent from the above, all or portions of the various systems,methods, and aspects of the present invention may be embodied inhardware, software, or a combination of both. When embodied in software,the methods and apparatus of the present invention, or certain aspectsor portions thereof, may be embodied in the form of program code (i.e.,instructions). This program code may be stored on a computer-readablemedium, such as a magnetic, electrical, or optical storage medium,including without limitation a floppy diskette, CD-ROM, CD-RW, DVD-ROM,DVD-RAM, magnetic tape, flash memory, hard disk drive, or any othermachine-readable storage medium, wherein, when the program code isloaded into and executed by a machine, such as a computer or server, themachine becomes an apparatus for practicing the invention. A computer onwhich the program code executes will generally include a processor, astorage medium readable by the processor (including volatile andnon-volatile memory and/or storage elements), at least one input device,and at least one output device. The program code may be implemented in ahigh level procedural or object oriented programming language.Alternatively, the program code can be implemented in an assembly ormachine language. In any case, the language may be a compiled orinterpreted language. When implemented on a general-purpose processor,the program code may combine with the processor to provide a uniqueapparatus that operates analogously to specific logic circuits.

Moreover, the invention can be implemented in connection with anycomputer or other client or server device, which can be deployed as partof a computer network, or in a distributed computing environment. Inthis regard, the present invention pertains to any computer system orenvironment having any number of memory or storage units, and any numberof applications and processes occurring across any number of storageunits or volumes, which may be used in connection with processes forimproving image processing in accordance with the present invention. Thepresent invention may apply to an environment with server computers andclient computers deployed in a network environment or distributedcomputing environment, having remote or local storage. The presentinvention may also be applied to standalone computing devices, havingprogramming language functionality, interpretation and executioncapabilities for generating, receiving and transmitting information inconnection with remote or local services.

Distributed computing facilitates sharing of computer resources andservices by exchange between computing devices and systems. Theseresources and services include, but are not limited to, the exchange ofinformation, cache storage, and disk storage for files. Distributedcomputing takes advantage of network connectivity, allowing clients toleverage their collective power to benefit the entire enterprise. Inthis regard, a variety of devices may have applications, objects orresources that may implicate processing performed in connection with theimage processing using the methods of the present invention.

Although not required, the invention can be implemented via an operatingsystem, and/or included within application or server software thatoperates in accordance with the invention. Software may be described inthe general context of computer-executable instructions, such as programmodules, being executed by one or more computers, such as clientworkstations, servers or other devices. Generally, program modulesinclude routines, programs, objects, components, data structures and thelike that perform particular tasks or implement particular abstract datatypes. Typically, the functionality of the program modules may becombined or distributed as desired in various embodiments. Moreover, theinvention may be practiced with other computer system configurations andprotocols. Other well known computing systems, environments, and/orconfigurations that may be suitable for use with the invention include,but are not limited to, personal computers (PCs), automated tellermachines, server computers, hand-held or laptop devices, multi-processorsystems, microprocessor-based systems, programmable consumerelectronics, network PCs, appliances, lights, environmental controlelements, minicomputers, mainframe computers and the like.

Those skilled in the art will appreciate that other image processingfeatures besides those specifically described herein may be used withthe techniques described herein. Such variations are intended to beincluded within the scope of the invention as defined by the followingclaims.

What is claimed:
 1. A method of modifying the histogram of a grayscaleimage to improve contrast, comprising: a processor performing the stepsof: extracting black connected components from the grayscale image thattouch at least one of the margins of the grayscale image; computing thehistogram of the portion of the grayscale image covered by the extractedblack connected components; and updating the histogram of the grayscaleimage by subtracting the histogram of the portion of the binary imagecovered by the extracted black connected components from the histogramof the grayscale image or by subtracting a function of number of pixelsof the portion of the binary image covered by the extracted blackconnected components from the histogram of the grayscale image.
 2. Amethod as in claim 1, further comprising binarizing the grayscale imageusing a predefined threshold to get a binary image and extracting theblack connected components from the binary image.
 3. A method as inclaim 1, wherein the function is a property of a document containing thegrayscale image.
 4. A method as in claim 3, wherein the function is thesize of the document whereby pixels outside of the document boundariesare removed from the histogram during the updating of the histogram. 5.A method as in claim 1, further comprising adjusting the contrast of thegrayscale image using a contrast adjustment program in which the p_(max)and p_(min) for the pixels of the contrast adjusted image are calculatedfrom the updated histogram h(i) as follows:$p_{\min} = {{\underset{0 \leq p \leq {R - 1}}{\text{argmax}}p} \in \{ {x❘{{\sum\limits_{i = 0}^{x}{h(i)}} \leq ( {c\mspace{11mu}{\% \cdot N}} )}} \}}$$p_{\max} = {{\underset{0 \leq p \leq {R - 1}}{\text{argmin}}p} \in \{ {x❘{{\sum\limits_{i = x}^{R - 1}{h(i)}} \leq ( {c\mspace{11mu}{\% \cdot N}} )}} \}}$where c is a given percentage value, N is the total image pixel numberand R is the grayscale resolution.
 6. A method as in claim 5, whereinthe contrast adjustment I_(adj) is calculated for each pixel (i,j) as:${I_{adj}( {i,j} )} = \{ \begin{matrix}0 & {{I( {i,j} )} \leq p_{\min}} \\\frac{( {R - 1} )( {{I( {i,j} )} - p_{\min}} )}{p_{\max} - p_{\min}} & {p_{\min} < {I( {i,j} )} < p_{\max}} \\{R - 1} & {{I( {i,j} )} \geq p_{\max}}\end{matrix} $ whereby c % of image pixels are saturated at lowand high intensities.
 7. A computer readable storage medium that is nota transient signal, said computer readable storage medium havinginstructions stored thereon that when processed by a processor cause theprocessor to implement a method of modifying the histogram of agrayscale image to improve contrast by performing the steps of:extracting black connected components from the grayscale image thattouch at least one of the margins of the grayscale image; computing thehistogram of the portion of the grayscale image covered by the extractedblack connected components; and updating the histogram of the grayscaleimage by subtracting the histogram of the portion of the binary imagecovered by the extracted black connected components from the histogramof the grayscale image or by subtracting a function of number of pixelsof the portion of the binary image covered by the extracted blackconnected components from the histogram of the grayscale image.
 8. Amedium as in claim 7, further comprising instructions for causing theprocessor to binarize the grayscale image using a predefined thresholdto get a binary image and to extract the black connected components fromthe binary image.
 9. A medium as in claim 7, wherein the function is aproperty of a document containing the grayscale image.
 10. A medium asin claim 9, wherein the function is the size of the document wherebypixels outside of the document boundaries are removed from the histogramduring the updating of the histogram.
 11. A medium as in claim 7,further comprising instructions for causing the processor to adjust thecontrast of the grayscale image using a contrast adjustment program inwhich the p_(max) and p_(min) for the pixels of the contrast adjustedimage are calculated from the updated histogram h(i) as follows:$p_{\min} = {{\underset{0 \leq p \leq {R - 1}}{\text{argmax}}p} \in \{ {x❘{{\sum\limits_{i = 0}^{x}{h(i)}} \leq ( {c\mspace{11mu}{\% \cdot N}} )}} \}}$$p_{\max} = {{\underset{0 \leq p \leq {R - 1}}{\text{argmin}}p} \in \{ {x❘{{\sum\limits_{i = x}^{R - 1}{h(i)}} \leq ( {c\mspace{11mu}{\% \cdot N}} )}} \}}$where c is a given percentage value and N is the total image pixelnumber, and R is the grayscale resolution.
 12. A medium as in claim 11,wherein the instructions cause the processor to calculate contrastadjustment I_(adj) for each pixel (i,j) as:${I_{adj}( {i,j} )} = \{ \begin{matrix}0 & {{I( {i,j} )} \leq p_{\min}} \\\frac{( {R - 1} )( {{I( {i,j} )} - p_{\min}} )}{p_{\max} - p_{\min}} & {p_{\min} < {I( {i,j} )} < p_{\max}} \\{R - 1} & {{I( {i,j} )} \geq p_{\max}}\end{matrix} $ whereby c % of image pixels are saturated at lowand high intensities.
 13. A device adapted to modify the histogram of agrayscale image to improve contrast, comprising: a scanner that scans animage to obtain said grayscale image; and a processor adapted to extractblack connected components from the grayscale image that touch at leastone of the margins of the grayscale image, to compute the histogram ofthe portion of the grayscale image covered by the extracted blackconnected components, and to update the histogram of the grayscale imageby subtracting the histogram of the portion of the binary image coveredby the extracted black connected components from the histogram of thegrayscale image or by subtracting a function of number of pixels of theportion of the binary image covered by the extracted black connectedcomponents from the histogram of the grayscale image.
 14. A device as inclaim 13, wherein the processor is further adapted to binarize thegrayscale image using a predefined threshold to get a binary image andto extracting the black connected components from the binary image. 15.A device as in claim 13, wherein the function is a property of adocument containing the grayscale image.
 16. A device as in claim 15,wherein the function is the size of the document whereby pixels outsideof the document boundaries are removed from the histogram during theupdating of the histogram.
 17. A device as in claim 13, wherein theprocessor is further adapted to adjust the contrast of the grayscaleimage using a contrast adjustment program in which the p_(max) andp_(min) for the pixels of the contrast adjusted image are calculatedfrom the updated histogram h(i) as follows:$p_{\min} = {{\underset{0 \leq p \leq {R - 1}}{\text{argmax}}p} \in \{ {x❘{{\sum\limits_{i = 0}^{x}{h(i)}} \leq ( {c\mspace{11mu}{\% \cdot N}} )}} \}}$$p_{\max} = {{\underset{0 \leq p \leq {R - 1}}{\text{argmin}}p} \in \{ {x❘{{\sum\limits_{i = x}^{R - 1}{h(i)}} \leq ( {c\mspace{11mu}{\% \cdot N}} )}} \}}$where c is a given percentage value, N is the total image pixel numberand R is the grayscale resolution.
 18. A device as in claim 17, whereinthe processor is further adapted to calculate contrast adjustmentI_(adj) for each pixel (i,j) as:${I_{adj}( {i,j} )} = \{ \begin{matrix}0 & {{I( {i,j} )} \leq p_{\min}} \\\frac{( {R - 1} )( {{I( {i,j} )} - p_{\min}} )}{p_{\max} - p_{\min}} & {p_{\min} < {I( {i,j} )} < p_{\max}} \\{R - 1} & {{I( {i,j} )} \geq p_{\max}}\end{matrix} $ whereby c % of image pixels are saturated at lowand high intensities.